Conventionally, a video camera which records motion images on a magnetic tape medium by using an image sensing means such as a CCD (Charge-Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor) and a digital still camera which records still images on a solid memory medium by using a similar image sensing means have been produced. Recently, as the capacity of the solid memory medium is increased and the speed and performance of an image signal processor of a photographing apparatus are improved, photographing apparatuses having functions of both motion image photographing and still image photographing are also beginning to be widely used. On the other hand, as the interval between unit light-receiving portions, that is, a so-called pixel pitch of an image sensing means is decreased, the influence of diffraction caused by a stop aperture for light amount control which a photographing optical system has is no longer negligible. That is, when the size of the stop aperture decreases in high-luminance object photographing or slow-shutter photographing, the image contrast lowers by the influence of so-called small-aperture diffraction. This makes it impossible to well achieve the original image forming performance of the photographing optical system or the high pixel capability of the image sensing means, so no high-resolution images can be obtained. To alleviate this small-aperture diffraction, therefore, a technique using a neutral density filter (to be referred to as an ND filter hereinafter) for attenuating the transmitted light amount is disclosed.
When the ND filter is used for this purpose, the naturalness of blurred images and the resolution and continuity of light amount control are also important as well as the original purpose, that is, the prevention of diffraction. To solve this problem, the following techniques are disclosed.
For example, Japanese Utility Model Laid-Open No. 51-145929 discloses a stop mechanism which uses ND filters having gradation as an appropriate number of aperture blades of an iris diaphragm made up of a large number of aperture blades. In this stop mechanism, the ND filters enter into a light beam after light-shielding blades are stopped down to the minimum aperture diameter, thereby controlling the light amount.
Japanese Patent Laid-Open No. 6-273655 discloses a lens barrel which includes a turret diaphragm and a turret ND filter, and controls the light amount by individually driving these members by using different actuators.
Japanese Patent Laid-Open No. 8-43878 discloses a stop device in which an ND filter whose transmittance decreases outward in the radial direction from the optical axis is formed for each of two aperture blades, and the light amount is controlled by driving the two aperture blades relative to each other.
Japanese Patent Laid-Open No. 11-218797 discloses an exposure control mechanism in which a turret aperture plate and ND filter are stacked in the optical axis direction, and these two members are driven by a single motor.
Japanese Patent Laid-Open No. 2000-155352 discloses a stop device in which the aperture diameter is controlled by operating a large number of aperture blades having the same shape in accordance with the operation of an actuator, and the light amount is controlled by inserting an ND filter into the aperture in a region in which the aperture diameter is changed little by the aperture blades.
Japanese Patent Laid-Open No. 2000-106649 discloses an apparatus which independently controls driving of iris blades for forming a stop aperture and an ND filter having a multilevel density.
Unfortunately, the above conventional techniques have the following drawbacks.
In the conventional techniques disclosed in Japanese Utility Model Laid-Open No. 51-145929 and Japanese Patent Laid-Open No. 2000-155352, the ND filters enter into a light beam after the light-shielding blades are stopped down to the minimum aperture diameter. This eliminates the degree of freedom of control of the ND filters. In particular, if the ND filters suddenly enter when a dark scene changes to a bright scene during motion image photographing, the continuity of exposure control deteriorates. This significantly degrades the naturalness of the reproduced motion image.
In the conventional technique disclosed in Japanese Patent Laid-Open No. 6-273655, the aperture is made up of a plurality of holes formed in a turret plate, so the F-number cannot be finely adjusted. In the second embodiment, a spiral hole having a continuously changing aperture width is used as an aperture. However, a circular stop aperture can not be obtained by this mechanism, so blurred images have unnatural shapes.
In the conventional technique disclosed in Japanese Patent Laid-Open No. 8-43878, the ND filter enters into the aperture in an open aperture state. This produces a light amount loss at open aperture. Also, since the aperture blades and ND filter are driven together, the degree of freedom of control of the ND filter is lost.
In the conventional technique disclosed in Japanese Patent Laid-Open No. 11-218797, the ND filter having a uniform density completely covers the stop aperture or does not cover the stop aperture at all. Therefore, no multi-step light amount control by the ND filter can be performed.
In the conventional technique disclosed in Japanese Patent Laid-Open No. 2000-106649, the rhombic shape of the stop aperture makes the shapes of blurred images unnatural. Also, since the strip-like ND filter requires a large accommodating space, the size of the whole apparatus increases.
On the other hand, in a photographing apparatus having functions of both motion image photographing and still image photographing, the image sizes (the numbers of photographing pixels) of acquired motion images and still images are generally different. That is, low-pixel images are continuously acquired during motion image photographing, and one to about a few frames of high-pixel images are acquired during still image photographing. In this case, even when the stop aperture diameter, that is, the F-number of a photographing optical system remains the same, small-aperture diffraction affects low-pixel motion images and high-pixel still images to different degrees. Therefore, techniques in which the forms of stop aperture control for motion image photographing and still image photographing are different are disclosed as follows.
In Japanese Patent Laid-Open No. 2002-64745, only a normal aperture range (e.g., the range of F2.8 to F8) having guaranteed optical performance is used in normal photographing (of still images). That is, the use of an open aperture having a large aberration is limited, or the use of a small aperture which increases the influence of diffraction is limited, thereby preventing deterioration of the image quality and obtaining high-resolution images. Also, when thinned-out photographing (of motion images) is to be performed, low-pixel images need only be acquired. Since this increases the allowance of optical aberration, a wide aperture range (e.g., the range of F1.4 to F11) by which the optical performance deteriorates is used to widen the exposure control range.
Likewise, in Japanese Patent Laid-Open No. 2002-204390, the F-number control range is limited from F1.4 to F16 during motion image photographing, and limited from F2.8 to F8 during still image photographing, thereby obtaining the same effect as in Japanese Patent Laid-Open No. 2002-64745 described above.
Unfortunately, the above conventional techniques still have the following drawbacks.
In the conventional technique disclosed in Japanese Patent Laid-Open No. 8-43878, the ND filter enters the aperture in an open aperture state. This produces a light amount loss at open aperture. Also, the density distribution of the ND filter in the vertical direction of the stop aperture differs from that in the horizontal direction of the stop aperture. This makes a blurred image unnatural when an object having a large distance difference is photographed. Accordingly, this structure is particularly unsuited to still image photographing requiring high-quality images.
Japanese Patent Laid-Open Nos. 2002-64745 and 2002-204390 disclose the techniques of performing optimum aperture control for each of motion image photographing and still image photographing. However, no light amount control using an ND filter is disclosed, and the prevention of small-aperture diffraction is unsatisfactory.
The conventional technique disclosed in Japanese Patent Laid-Open No. 2000-106649 has a degree of freedom of transmittance control by a stop aperture diameter and ND filter. However, details of switching between the control forms of motion image photographing and still image photographing are not described.